Soil cores were collected from three sites within the Plum Island Ecosystems - Long Term Ecological Research (PIE-LTER) domain (MA, USA; 42.74° N, -70.85° W). A core liner was fitted with a gasketed piston and placed on the sediment surface and pushed down into the marsh subsurface, ensuring that the soil column did not compact during collection.

The sites had similar elevations (1.41 - 1.51 m North American Vertical Datum of 1988 [NAVD88]) and salt marsh grass communities, dominated by Spartina patens, S. alterniflora, and Distichlis spicata. Permanently inundated ponds within each site had comparable depths (0.24 - 0.30 m) but varied in size (643 - 7,149 m²) and age (40 - 53 years) (Spivak et al., 2017; Spivak et al., 2018).

Soil cores were split lengthwise and sectioned into 1, 2, or 5 cm sections, with higher resolution in the top 30 cm. Soil water content (%) and bulk density (g/cm³) were determined gravimetrically after drying to constant mass (60°C). Samples were sieved (1 mm) to remove root material and homogenized with a Retsch Mixer Mill 200.

The thermal reactivity of SOC was characterized through ramped pyrolysis oxidation (RPO). Homogenized soil samples (~6 mg) were exposed to a constant flow of ultra-high purity oxygen and helium gas mixture within a reactor, where temperatures ramped to 1000°C at a constant rate (20°C per min) (Rosenheim et al., 2008). The evolved carbon dioxide (CO2) was measured by a flow-through infrared gas analyzer and thermograms were constructed by plotting gas concentrations over time. Thermograms were normalized to the expected milligram of carbon evolved per gram of dry soil sample (mg C / g soil). Samples were not acidified as inorganic carbon content was 0.02% - 0.04% of TOC. Analyses were conducted at the National Ocean Sciences Accelerator Mass Spectrometry Facility (NOSAMS, Woods Hole, MA). We further analyzed the δ13C and Fm composition of CO2 evolved from marsh and pond soils at one representative site. Refer to Hemmingway et al. (2017) for a description of the NOSAMS RPO set up.

RPO_fast.csv